Sulfur adsorption



United States Patent O 3,416,293 SULFUR ADSORPTION Larry G. Alexander,Louisville, Ky., assignor to Catalysts and Chemicals, Inc., acorporation of Delaware No Drawing. Filed June 22, 1967, Ser. No.647,907 Claims. (Cl. 5573) ABSTRACT OF THE DISCLOSURE The use ofactivated carbon in removing small quantities of sulfur compounds hasgained wide acceptance as a pretreatment stage in hydrocarbon streamsused in such processes as steam gas reforming. Capacity of activatedcarbon or metal oxide carbon is increased by the use of water.

BACKGROUND OF THE INVENTION It has long been the practice to removesulfur compounds by means of activated carbon from natural andmanufactured gases, ammonia, hydrogen and from LPG and varioushydrocarbon gases, prior to using such streams in chemical reactions. Inaddition the commercial acceptance of steam-hydrocarbon reforming forhigh purity hydrogen production focused attention on the need forremoving sulfur from a natural gas feed stream.

The use of activated carbon for removing small quantities of sulfur orsulfur compounds from a natural gas stream began to be practiced in thelate 1940"s. The introduction of chemically treated activated carbons inthe 1950's improved the effectiveness of the carbon, and enabled thismethod of desulfurization to gain wide acceptance.

Today the use of activated carbon as a pretreatment process to removetrace quantities of sulfur from natural gas and LPG gas streams is widespread. Both activated carbon and activated carbon which has beentreated or combined with various metal oxides are used. Metal oxides incombination with carbon appear to produce a synergistic effect whichgreatly improves the capacity of the activated carbons for theadsorption of sulfur compounds. Such metals as copper, iron, manganese,nickel, cobalt, cadmium, zinc, and the like are used in amounts of 3,to, say, percent by weight based on the carbon, the carbon usually beingimpregnated with a solution of a metal salt which can be heat decomposedto the oxide. Activated carbon and activated carbon modified withvarious metal oxides are an expedient and economical means ofdesulfurization. They are primarily used to remove mercaptans,disulfides, thioethers, thiophens and hydrogen sulfide present in gasstreams in amounts less than three volume percent, normally amounts of10 p.p.m. to 0.5 percent. Activated carbon is well known, and has hadmany uses because of its high adsorption capacity, usually beingcharacterized by a high surface area, say above 500 and generally above1000 square meters per gram.

In operation the carbon is placed in fairly large diameter vessels ordrums, two drums normally being used in parallel so that one can put onstream while the other one is being regenerated. Most uses of the drumsare based on five days on stream, followed by a one day regeneration. Indesulfurizing applications the carbon drums operate at ambienttemperatures, i.e., 32 F. to 150 F., and at the pressure employed in theplant, which is controlled by other processing requirements, generally apressure between 10 and 1000 p.s.i.g., and a space velocity of 500 to6000 volumes of gas per volume of carbon.

SUMMARY OF INVENTION Heretofore when steam has been used to purge theadsorbent vessel prior to operation, or during regeneration,

3,416,293 Patented Dec. 17, 1968 DETAILED DESCRIPTION OF THE INVENTIONIn its preferred embodiment this invention contemplates incorporatingwater in the catalyst as a result of steam regeneration. However it isalso possible to bring the adsorbent to its water content by steam purgeor by the presence of water vapor in a process gas purge at any stageprior to putting the adsorbent on stream.

The effect of bringing the carbon adsorbent to a selected water contentprior to its use in desulfurization is best illustrated by the followingexamples. These examples are, of course, illustrative only since variousmethods of introducing water will occur to those skilled in the art.

In these examples one hundred fifty liters per hour of a natural gasstream containing 3.3 p.p.m. H 8, 0.5 p.p.m. RSH, 0.3 p.p.m. RSR and 0.8p.p.m. residual sulfur believed to be COS and RSSR, was passed through a50 cubic centimeter catalyst bed. The adsorbent capacity was rated asliters of gas per liter of adsorbent which it passes through without anysulfur leakage, in other words without any sulfur passing through asdetermined by a coulombmetric sulfur analyzer. The gas was a natural gasstream containing about 5 p.p.m. mercaptan, and was passed through theadsorbent at a temperature in the range of 35 F. to F. The followingdata was obtained by variations in regeneration procedures using a zincoxide modified activated carbon adsorbent.

As the examples will show, it has been found that too much water isdetrimental to the adsorbent. Accordingly, there is an optimum amountwhich should be present. Usually the adsorbent should contain at leastthree percent water based on its own weight and generally not over sixtypercent. Preferably more water will be used in the case of activatedcarbon which is not modified. Thus in the case of metal oxide modifiedactivated carbons desirably the water content will be in the lower halfof the range, whereas in the case of activated carbon which is notmodified with a metal oxide, the water content best will be in the upperhalf of the range.

Example 1 An adsorbent saturated with sulfur compounds to capacityduring use was regenerated with steam at a temperature of 500 to 750 F.until the quantity of sulfur pres cut was negligible. The adsorbent wasthen purged with recycled natural gas until it cooled to ambienttemperature, the recycle gas driving the steam out of the adsorbent atthe high initial purging temperatures by the time the system cooleddown.

Example 2 An adsorbent saturated as in Example 1 was regenerated withsteam at a temperature of 465 F. maintaining steam flow until thetemperature of the adsorbent reached the steam temperature and sulfurwas gone. The unit was then shut down under pressure and permitted tostand until it cooled to ambient temperature. Subsequent purging withrecycle gas showed the adsorbent to be saturated with water.

Example 3 An adsorbent was regenerated as in Example 1 with 470 F. steamuntil the temperatures were equalized and the sulfur gone. The steam wasthen depressured, lower temperature steam being gradually introduceduntil the temperature of the system reached 200 F. When the temperaturereached 200 F. the unit was shut down and permitted to cool further tothe ambient temperature.

In the following table are set forth adsorption capacities of theadsorbents treated in accordance with the foregoing examples.

Table 1 Capacity (liters of gas per Regeneration method: liter ofadsorbent) Example l-No water vapor present 44,520 Example 2Saturatedwith water 50,040 Example 3Contained optimum amount of water 280,000

Example 4 As in Example 1 the adsorbent was regenerated with steam at460 F. After regeneration steam was depressured to bring the systemtemperature to 250 F. The unit was then cooled to ambient temperaturewith recycle gas.

Example 5 The adsorbent of Example 4 was subsequently regenerated at 290F. with steam and then cooled as set forth in Example 4.

Example 6 When the adsorbent employed in Example 5 had again reached itscapacity of sulfur adsorption, it was again regenerated as in Example 5,the temperature being 270 F.

Example 7 The adsorbent employed in Example 6 was regenerated at 295 F.when it had again reached its sulfur capacity. The regenerationprocedure being the same as that employed in Example 5.

In Table 2 are given adsorption capacity values wherein the adsorbentwas cooled with a process gas rather than being permitted to cool onstanding. The table also shows the effect of subsequent regenerations ofthe same adsorbent.

Table 2 Capacity gas liters liter Regeneration method: of adsorbentExample 41st regeneration 94,490 Example 5-2nd regeneration 129,780Example 63rd regeneration 185,700 Example 74th regeneration 281,460

As can be seen from Table 2 whereas the first regeneration does notincrease the capacity as much as shutting down, the regenerations havebeen found to be cumulative. Thus the capacity following eachregeneration is better than that following the regeneration before it.

Equivalent results are obtained with carbon modified with about 10percent, as metal, cobalt and iron (C00 and FeO). Thus a capacity of300,000 standard cubic feet of gas per cubic foot of carbon wereobtained for cobalt by the invention. This compares with 60,000 cubicfeet per cubic foot of carbon when no water is present. An adsorbentcarbon devoid of metal oxides had a capacity of 20,000 liters per literof carbon compared with 60,000 when the carbon contained 30 percentwater. The gas in each instance was a percent methane 10 percenthydrogen stream containing ppm. hydrogen sulfide.

This invention thus provides an excellent method for improving theadsorption capacities of activated carbons. As has been shown the Watercan be included at any stage of use, and it can be added without havingto shut down the unit in order to cool it. Given the foregoingillustrations other modifications will occur to one skilled in the art.Such modifications are deemed to be within the scope of this invention.

What is claimed is:

1. In the process for removing sulfur compounds from industrial gasstreams by adsorption by passing the gas stream through a bed containingan activated carbon adsorbent the improvement for increasing theadsorption capacity of the activated carbon adsorbent for mcrcaptans,disulfides, thioethers, thiophenes and hydrogen sulfide present in gasstreams which comprises introducing onto the porous surfaces of theadsorbent a quantity of water vapor such that the carbon adsorbent asthe desulfurization commences contains at least 3 weight precent and notover 30 weight percent Water based on the adsorbent, the adsorbent beingselected from the group consisting of activated carbon modified by ametal selected from the group consisting of copper, iron, manganese,nickel, cobalt, cadmium, and zinc.

2. The process of claim 1 wherein the activated carbon is modified withzinc oxide.

3. The process of claim 1 wherein the activated carbon is modified withcopper oxide.

4. The process of claim 1 wherein the activated carbon is modified withiron oxide.

5. The process of claim 1 wherein the water vapor is introduced onto thecarbon by condensing steam thereon during regeneration.

6. The process of claim 1 wherein the water vapor is introduced onto theactivated carbon prior to its initial use.

7. In the process for removing sulfur compounds from industrial gasstreams by absorption by passing the gas stream through a bed containingactivated carbon as an absorbent the improvement for increasing theabsorption capacity of the activated carbon adsorbent for mercaptans,disulfides, thioethers, thiophene's and hydrogen sulfide present in gasstreams which comprises introducing into the porous surfaces of theabsorbent carbon a quantity of water vapor such that the absorbentcarbon as the desulfurization commences contains at least 3 weightpercent and not over 60 weight percent water based on the absorbent.

8. The process of claim 7 wherein the water vapor is introduced onto theadsorbent during regeneration.

9. The process of claim 7 wherein the sulfur containing gas is naturalgas.

10. The process of claim 7 wherein the gas stream is passed through thecarbon bed in several stages, water being added between stages.

References Cited UNITED STATES PATENTS 5/1951 Robinson 23-2.1 8/1965Fujita et al 233.l

U.S. Cl. X.R.

